1. Field of the Invention
The present invention relates to a method for manufacturing a hot-rolled steel sheet with high strength and high formability applicable to automobiles, industrial machine and the like. More specially, this invention relates to a method for manufacturing a hot rolled TRIP (Transformation Induced Plasticity) steel containing copper (Cu) with high strength and high ductility.
2. Description of the prior art
Generally a high strength hot-rolled steel sheet with high formability has been widely used in manufacturing automobiles. And order to make the automobile steel sheet lighter and ensure safety at collisions, steel sheets with a higher strength have been greatly demanded.
This kind of hot-rolled high strength steel sheet is widely used in making automobile driving wheels. The effect of weight reduction in the components used in the driving system, like driving wheels, is higher by 3 times than the body panel. Further, the weight reduction greatly depends on the high strength, and therefore, a high strength steel sheet is increasingly demanded.
Since the components such as driving wheels are formed into the final product through a complicated press-forming procedure, the steel used for them has been required to have superior formability.
Research has been encouraged to produce new types of hot rolled steels with higher strength without significant loss so ductility. As a result, dual phase steel composed of either ferrite and martensite or ferrite and bainite, and triphase steel composed of ferrite, martensite and bainite were developed, the strength of which reaches 60 kg/mm.sup.2 and the elongation 30%.
The methods for producing these steels have been proposed as described in the following examples;
1. A steel containing 0.06-0.1% by weight of C, 0.25-1.3% by weight of Si, and 1.1-1.5% by weight of Mn is coiled at a temperature of 300.degree. C. or below, thereby producing a dual phase steel composed of ferrite and martensite (Testu-to-Hagane, vol. 68(1992), p.1306).
2. A steel containing 0.04-0.06% by weight of C, 0.5-1.0% by weight of Si, and 1.5% by weight of Mn, is added with 0.5-1.5% by weight of Cr. This steel is rolled at about 850.degree. C., then coiled at about 200.degree. C. Thus a triphase steel is produced, of which the microstructure consists of 10-20% by volume of bainite and 3-5% by volume of martensite in ferrite matrix. (Testu-to-Hagane, vol. 68(1992), p.1185).
3. A steel containing 0.05-0.07% by weight of C, 0.5% or less by weight of Si, 1.1-1.5% by weight of Mn, is added with 0.04% or less by weight of Nb. Dual phase steel containing 1-20% by volume of bainite phase in ferrite matrix is produced, and in which the tensile strength is the order of 60 kg/mm.sup.2. (Trans. ISIJ, vol.23(1983), p.303).
4. A steel having compositions similar to that of item 3 is added with 0.04% and 0.06% by weight of Nb and Ti based on the item 3. The ferrite-bainite dual phase steel with tensile strength of 70 kg/mm.sup.2 is produced. (CAMP-ISIJ, vol.1(1988) p.881).
If the strength of the aforementioned steels is improved, however, the ductility is drastically decreased. For example, if the tensile strength is enhanced to 90 kg/mm.sup.2, the ductility is dropped to less than 20%. The formability, therefore, is drastically aggravated.
Using the concept of TRIP (Transformation Induced Plasticity) phenomena observed in austenitic steel, however, the combination of high strength and high formability can be obtained. Thus, the steels containing retained austenite possess superior combination of high strength and high formability owing to the strain induced transformation of retained austenite to martensite during deformation.
If the process conditions are optimized, the steel containing retained austerite shows a good combination of strength and ductility with a tensile strength up to 80 kg/mm.sup.2 and an elongation of 30%. In this regard, various techniques have been proposed.
1. Japanese patent laid-open No. Hei-6-145892 discloses a steel containing 0.06-0.22% by weight of C, 0.05-1.0% by weight of Si, 0.5-2.0% by weight of Mn, 0.25-1.5% by weight of Al, and 0.03-0.3% by weight of Mo. Thus the volume fraction of retained austenite reaches 3-20%, and a tensile strength of 50 kg/mm.sup.2 and an elongation of 35% are obtained. The steel shows a prominent press formability, high deep drawability, and superior bendability.
2. Japanese patent laid-open No. Hei-6-145788 discloses a steel in which the Al content of the steel of Japanese patent laid-open No. Hei-6-145892 is adjusted to the range of 0.6%.times.Si and 3-12.5%.times.C. The steel is annealed at 600-950.degree. C. for 10 seconds to 3 minutes, which is ferrite/austenite two phase region, cooled down to 350-600.degree. C. at a cooling raze of 4-200.degree. C./sec, and isothermally held at this temperature for 5 seconds to 10 minutes. The steel, in turn, is cooled down to below 250.degree. C. at a cooling rate of 5.degree. C./sec or more, thereby obtaining steels with high formability.
3. Japanese patent laid-open No. Sho-62-188729 discloses that a steel containing 0.15-0.3% by weight of C, 0.5-2.0% by weight of Si, 0.2-2.5% by weight of Mn, 0.1% or less by weight of Al, and 0.05-0.5% by weight of Cr (if necessary) is annealed ferrite/austenite two phase region (730-920.degree. C.) for 20 seconds to 5 minutes, cooled down to a temperature of 650-770.degree. C. at a cooling rate of 2-50.degree. C./sec, isothermally held at this temperature for 5 seconds to 1 minutes, and then cooled down to a temperature of 300-450.degree. C. at a cooling rate of 10-500.degree. C./sec. By following up the above procedure, a steel with a tensile strength of 60 kg/mm.sup.2 or more with good formability is obtained.
4. Japanese patent laid-open No. Hei-4-228517 and Hei-4-228538 disclose a steel containing 0.15-0.4% by weight of C, 0.5-2.0% by weight of Si, 0.2-2.5% by weight of Mn which is subjected to a finish rolling at a temperature of Ar.sub.3.+-.50.degree. C., cooled down to a temperature of Ar.sub.1 at a cooling rate of 40.degree. C./sec, and cooled again down to a temperature of 350-400.degree. C. at a cooling rate of 40.degree. C./sec. By following up the above procedure, a steel of which uniform elongation is 20% or more and the value of TS.times.El. is 2,400 (kg/mm.sup.2.times.%) is obtained.
5. Japanese patent laid-open No. Hei-5-179396 discloses a steel containing 0.18% or less by weight of C, 0.5-2.5% by weight of Si, 0.5-2.5% by weight of Mn, 0.05% or less by weight of P, 0.02% or less by weight of S, and 0.01-0.1% by weight of Al. Additionally 0.02-0.5% by weight of Ti and 0.03-1.0% by weight of Nb can be added. The contents of Nb and Ti are adjusted to % C&gt;(% Ti/4)+(% Nb/8). The steel is finish-rolled at 820.degree. C. or above, held at a temperature of 820-720.degree. C. for 10 seconds or more, cooled down to 500.degree. C. or below at a cooling rate of 10.degree. C./sec, and coiled at this temperature. By following up the above procedure, a steel having high ductility, enhanced fatigue property, a spot weldability, and high strength (70 kg/mm.sup.2 or more) is obtained.
6. Japanese patent laid-open No. Hei-5-311323 discloses a steel containing 0.1-0.2% by weight of C, 0.8-21.6% by weight of Si, 3.0-6.0% by weight of Mn, 0.5% or less by weight of Al which is annealed at ferrite/austenite two phase region for 1-20 hours, and furnace cooled to let the volume fraction of retained austenite 10% or more. By following up the above procedure, a steel with the tensile strength of 80 kg/mm.sup.2 and superior formability is obtained.
7. Japanese patent laid-open No. Hei-5-112846 discloses a steel containing 0.05-0.25% by weight of C, 0.05-1.0% by weight of Si, 0.8-2.5% by weight of Mn, 0.8-2.5% by weight of Al, which is finish-rolled at a temperature of 780-840.degree. C., cooled down to a temperature of 600-700.degree. C. at a cooling rate of 10.degree. C./sec, air-cooled for 2-10 seconds, and then rapidly cooled down to a temperature of 300-450.degree. C. at a cooling rate of 220.degree. C./sec. By following the above procedure, a steel containing 5% or more by volume of retained austenite is obtained.
A precipitation hardened, hot-rolled dual phase steel has been developed. In this steel, a soft ferrite phase is effectively hardened by the precipitation, and has the tensile strength of 80 kg/mm.sup.2 and high ductility (Japanese Iron and Steel Newspaper dated Sep. 4, 1993).
The steels described above have been developed suitably for the intended use, and commercialized. They have a tensile strength of 90 kg/mm.sup.2 or less, and corresponding elongations. The hot-rolled steel sheets for use in automobiles are, however, increasingly required to have improved strength as well as good formability.